davoustp edited a comment on pull request #504:
URL: https://github.com/apache/struts/pull/504#issuecomment-966671933
Very good question, indeed. ;-)
This is because this single expression can be evaluated through a chain, and
the way the method `ognl.ASTChain.getValueBody(OgnlContext, Object)` is built
can lead to method `ognl.OgnlRuntime.getProperty(OgnlContext, Object, Object)`
throwing a `OgnlException("source is null for getProperty(null, \"" + name +
"\")")` (line 2733).
Hence the expression is not cached, before this code change.
Here are a few values of the`name` attribute of method
`ognl.OgnlRuntime.getProperty(OgnlContext, Object, Object)` creating this
scenario:
`
actionMapping
opensymphony
apache
valueStack
actiontag
servlet
Application__
Request__
`
And a stack trace for the {{actionMapping}} case (Struts 2.5.20):
`
Daemon Thread [http-nio-8080-exec-6] (Suspended (breakpoint at line 2733 in
OgnlRuntime))
owns: NioEndpoint$NioSocketWrapper (id=294)
OgnlRuntime.getProperty(OgnlContext, Object, Object) line: 2733
ASTProperty.getValueBody(OgnlContext, Object) line: 114
ASTProperty(SimpleNode).evaluateGetValueBody(OgnlContext, Object) line:
212
ASTProperty(SimpleNode).getValue(OgnlContext, Object) line: 258
ASTChain.getValueBody(OgnlContext, Object) line: 141
ASTChain(SimpleNode).evaluateGetValueBody(OgnlContext, Object) line: 212
ASTChain(SimpleNode).getValue(OgnlContext, Object) line: 258
Ognl.getValue(Object, Map, Object, Class) line: 470
Ognl.getValue(Object, Map, Object) line: 434
OgnlUtil$2.execute(Object) line: 401
OgnlUtil.compileAndExecute(String, Map<String,Object>, OgnlTask<T>)
line: 442
OgnlUtil.getValue(String, Map<String,Object>, Object) line: 399
OgnlValueStack.getValueUsingOgnl(String) line: 293
OgnlValueStack.tryFindValue(String) line: 276
OgnlValueStack.tryFindValueWhenExpressionIsNotNull(String) line: 258
OgnlValueStack.findValue(String, boolean) line: 238
OgnlValueStack.findValue(String) line: 300
StrutsRequestWrapper.getAttribute(String) line: 94
PrepareOperations.findActionMapping(HttpServletRequest,
HttpServletResponse, boolean) line: 181
PrepareOperations.findActionMapping(HttpServletRequest,
HttpServletResponse) line: 165
StrutsPrepareFilter.doFilter(ServletRequest, ServletResponse,
FilterChain) line: 90
ApplicationFilterChain.internalDoFilter(ServletRequest, ServletResponse)
line: 189
ApplicationFilterChain.doFilter(ServletRequest, ServletResponse) line:
162
...
`
Here, the Struts layer attempts to find the `actionMapping` to use, which is
first evaluated using OGNL onto the ValueStack - and in this specific case,
there is no such entry in the stack - before it actually gets into the struts
configuration to find the appropriate mapping.
I did spent some time to check how method
`ognl.ASTChain.getValueBody(OgnlContext, Object)` works, and I'm actually
puzzled to see that the `source` attribute is actually the result of the
previous step in the chain (which can be null, leading to the exception being
raised). But I'm not sure whether this is a problem or a by-design choice (and
I sure don't have enough background onto the OGNL internals to decide).
Would you say that this sequence of event is normal, or is it something that
should not occur?
Let me know, and I'll open another issue - I can reproduce it 100% so it's
easy to investigate if you find it useful...
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